Mass spectrometers have become common tools in chemical analysis. Generally, mass spectrometers operate by separating ionized atoms or molecules based on differences in their mass-to-charge ratio (m/e). A variety of mass spectrometer devices are commonly in use, including ion traps, quadrupole mass filters, and magnetic sector mass analyzers.
The general stages in performing a mass-spectrometric analysis are:
(1) create gas-phase ions from a sample; (2) separate the ions in space or time based on their mass-to-charge ratio; and (3) measure the quantity of ions of each selected mass-to-charge ratio. Thus, in general, a mass spectrometer system consists of means to ionize a sample, a mass-selective analyzer, and an ion detector. In the mass-selective analyzer, magnetic and electric fields may be used, either separately or in combination, to separate the ions based on their mass-to-charge ratio. Hereinafter, the mass-selective analyzer portion of a mass spectrometer system will simply be called a mass spectrometer. Mass spectrometers operate under vacuum conditions.
Accordingly, it is necessary to prepare the sample undergoing analysis for introduction into the vacuum environment of the mass spectrometer. This presents particular problems for high molecular weight compounds or other sample materials which are difficult to volatilize. While liquid chromatography is well suited to separate a liquid sample matrix into its constituent components, it is difficult to introduce the output of a liquid chromatograph (LC) into a mass spectrometer. One technique that has been used for this purpose is the "electrospray" method.
The electrospray or electrospray ionization technique may be used to produce gas-phase ions from a liquid sample matrix to permit introduction of the sample into a mass spectrometer. It is thus useful for providing an interface between a liquid chromatograph and a mass spectrometer. In the electrospray method, the liquid sample to be analyzed is pumped through a capillary tube or needle. A potential difference (of for example, three to four thousand volts) is established between the tip of the electrospray needle and an opposing wall, capillary entrance, or similar structure. The needle can be at an elevated potential and the opposing structure can then be grounded; or the needle can be at ground potential and the opposing structure can be at the elevated potential (and of opposite sign to the first case). The stream of liquid issuing from the needle tip is broken up into highly charged drops by the electric field, forming the electrospray. An inert gas, such as dry nitrogen gas (for example) may also be introduced through a surrounding capillary to enhance nebulization (droplet formation) of the fluid stream.
The electrospray drops consist of sample compounds in a carrier liquid and are electrically charged by the electric potential as they exit the capillary needle. The charged drops are transported in an electric field and injected into the mass spectrometer, which is maintained at a high vacuum. Through the combined effects of a heated counter flow of drying gas and vacuum, the carrier liquid in the drops starts to evaporate giving rise to smaller, increasingly unstable drops from which surface ions are liberated into the vacuum for analysis. The desolvated ions pass through a skimmer cone, and after focusing by an ion lens, into the high vacuum region of the mass spectrometer, where they are separated according to mass-to-charge ratio and detected by an appropriate detector (e.g., a photo-multiplier tube).
Although the electrospray method is very useful for analyzing high molecular weight samples in a carrier liquid, it does have some limitations. For example, commercially available electrospray devices utilizing only electrospray nebulization to form the spray are practically limited to liquid flow rates of 20-30 microliters/min, depending on the solvent composition. Higher liquid flow rates result in unstable and inefficient ionization of the dissolved sample. Since the electrospray needle is typically connected to a liquid chromatograph, this acts as a limitation on the flow rate of the chromatograph.
One method of improving the performance of electrospray devices at higher liquid flow rates is to utilize a pneumatically assisted electrospray needle. One example of such a needle is formed from two concentric, stainless steel capillary tubes. In such a device the sample-containing liquid flows through the inner tube and a nebulizing gas flows through the annular space between the two tubes. This improves the efficiency of the ionization process by improving the ability of the electrospray needle to form small drops from the sample liquid. However, at high sample liquid flow rates into this type of electrospray needle, the drops formed are of such large size that they can degrade the performance of the mass spectrometer (by increasing the noise) if allowed to enter the device. This makes such electrospray needles less desirable for use with liquid chromatographs, which typically have relatively high flow rates at their output.
The use of liquid sample matrices having a high percentage of water in the electrospray method is limited to very low flow rates; even when using pneumatically assisted electrospray techniques. This is because solutions with a high percentage of water are prone to unstable droplet formation, even at very low liquid flow rates. Low surface tension liquids are preferable for use in electrospray ionization since electrostatic dispersion of droplets occurs when coulomb forces exceed those due to surface tension. This situation is more difficult to achieve for water due to its extremely high surface tension (72 dyne/cm) compared to organic liquids such as methanol (24 dyne/cm). Adding a modifying liquid, such as methanol, to an aqueous liquid reduces the surface tension of the liquid and improves the efficiency of electrospray ionization. However, for many chromatographic applications, the addition of an organic modifier liquid to the mobile phase may impair the separation ability of the chromatography process.
The prior art discloses the use of a liquid sheath of modifying liquid which is made to flow outside of the electrospray needle, through which flows the liquid sample matrix. Such a configuration is shown in FIG. 1, in which electrospray needle 100 is surrounded by a tube 102 through which flows a modifying liquid 106. The annular flow of sheath liquid 105 flows to the end of needle 100 where it merges with the inner flow of liquid sample matrix 108. The output of electrospray needle 100 are charged liquid droplets 109.
The art also discloses the use of a liquid sheath of modifying liquid which is made to flow inside of the electrospray needle, which contains a second tube transporting the sample containing aqueous fluid. In this configuration, which is shown in FIG. 2, inner sample tube 101 is displaced inward away from the end of electrospray needle 100 to form a mixing volume 107 for liquid sample matrix 108 and modifying liquid 106.
However, while useful, both of the prior art approaches shown in FIGS. 1 and 2 have disadvantages. The prior art device shown in FIG. 1 has the disadvantage of not providing a means for mixing the two liquids. The outer sheath flow liquid flows over the electrospray needle and joins the inner flow of sample liquid. The flow of inner sample liquid and the outer annular modifying liquid are both laminar flows, so that there is no mixing of the two liquids. While outer liquid 106 with the lower surface tension efficiently forms small droplets, the inner core of high surface tension aqueous liquid containing the sample is inefficiently nebulized into large droplets. This leads to increased noise in the mass spectrometer data.
In addition, while the prior art device shown in FIG. 2 includes a mixing region, it is very inefficient in mixing the two liquids. This is because the modifying liquid moves in an annular flow concentric about the inner flow of sample liquid. Both liquids exhibit laminar flow and very little mixing beyond that in the region of adjacent liquids at the outside of the sample flow occurs in the short mixing volume of this device. Thus, this structure has similar disadvantages to that of the prior art device of FIG. 1.
What is desired is an apparatus to provide an improved method of mixing a modifying liquid into a liquid sample matrix which is flowing into an electrospray ionization source, in order to improve the efficiency of the electrospray ionization process. It is further desired to provide a means to periodically switch a calibration liquid into and out of the liquid sample matrix stream without undesirable carry-over of the calibration material into the electrospray source.